This invention concerns polymeric films and in particular films for forming labels for in-mold labelling.
In-mold labelling is a technique in which a pre-formed polymeric label is incorporated into a polymeric article by molding the article in a mold containing the label. This technique in general provides labelled articles having a good appearance with the advantage that the labels effectively become an integral part of the articles and are thereby difficult if not impossible to remove. However, the molding process in general involves the use of high temperatures to effect melting or softening of the polymer used to form the articles, and since the pre-formed labels are usually made from polymeric films which have been stretched in their production, the temperatures to which the labels are subjected during the molding process usually lead to their shrinkage.
Although the dimensional changes resulting from label shrinkage can often be tolerated, and indeed anticipated when the labels are produced, shrinkage can lead to the films acquiring an undesirable appearance. In particular, it has been found that labels made from polypropylene films having a voided or cavitated layer resulting from the presence of a voiding agent such as chalk acquire a non-uniform or leathery appearance when they shrink. This is particularly disadvantageous as bonding of the molded polymer to the labels is usually improved significantly if the surface of the label which contacts the polymer being molded is voided or cavitated. Furthermore, voided or cavitated films are often preferred for forming labels as they are usually opaque and provide good contrast for printed indicia on the labels.